Power supply noise, simultaneous switching noise, or dynamic switching noise, is one of the most significant noise problems in deep sub-micron integrated circuits. The problem stems from switching noise on the power supply lines that are coupled from other signal nodes.
The most effective way to reduce power supply noise, however, is to use decoupling capacitors to filter out noise coupling between a positive supply voltage and a complimentary lower supply voltage. Such power noises may be induced by transistors in a high density IC demanding a large current with a high frequency, which results in abrupt voltage drops. There can be both global and localized voltage drops on the power grid of the IC. This voltage drop can be reduced by providing localized current sources. For example, capacitors may be used to decouple current surges from the power grid, and thereby reducing noise on the power grid.
Existing decoupling capacitors have disadvantages of having lower capacitance per area and having substantial gate leakage. The capacitance density and leakage current are trade-off in general; MOS capacitors with thick gate-oxide typically usually have lower current leakage and lower capacitance density as well. Therefore, what is needed is a decoupling capacitor having reduced leakage, but that maintains high capacitance per area and compatibility with standard complementary metal-oxide-semiconductor (CMOS) cells. Since power supply voltage substantially reduces with technology scaling down, the MOS capacitor having thin gate-oxide has become a potential choice. The present disclosure addresses such a need.